Prebreakdown in low voltage varistors and its relation with deep levels

The non-linearity coefficient α of several low voltage varistors shows some local maxima in the prebreakdown region (where the leakage current is generally defined). The field at which these maxima appear is not thermally activated and is related to the presence of deep levels in the forbidden gap of the zinc oxide. These levels are studied by an admittance spectroscopy method based on loss tangent measurements as a function of temperature. A barrier model enabled us to derive the energy of the levels and the relative proportion of deep levels to donor levels. The increase of non linearity below the main threshold can be explained by an abrupt jump of the barrier height when the Fermi level crosses the deep level of the trap inside the grain.